The present invention relates generally to systems for delivering power from an electrical source to an electric load, especially where there is relative motion between the source and the load, and more particularly to an improved contactless power delivery system, an improved contactless power distribution system, and methods for accomplishing this power transfer without contact of primary and secondary windings, such as may be used in electric vehicles, elevators, temporary work sites, underwater applications and the like. Also on Sep. 27, 1991, the same inventors filed U.S. patent application Ser. No. 07/766,756, which issued on Oct. 20, 1992, as U.S. Pat. No. 5,157,319, entitled "Contactless Battery Charging System." The '319 patent discloses a contactless recharging system and method for charging an energy storage device, such as one on-board an electric vehicle, using a coupling link, which may be constructed in one of the forms described further below.
The problem of supplying energy to electric vehicles while they are moving on a roadway has been considered difficult to solve. In the past, systems transferring large amounts of electrical power, such as up to one megawatt, to a moving load have traditionally used means that are unreliable, inefficient, and potentially unsafe, such as sliding or rolling metal contacts, sliding carbon brushes, and trailing cables. For example, pantograph sliding contacts have been used extensively for trolleys and urban transit systems traveling on rails. However, these railway traction systems are not a viable consideration for electric vehicles designed for consumer use because the public has access to the roadways and would be endangered by the exposed contacts. Additional problems include wear, corrosion, contact bounce and thermal cycling, all of which result in poor connection between the moving contacts. Furthermore, the reliability and maintenance problems of sliding mechanical contact systems significantly increase the initial capital costs of installation and the maintenance costs thereafter.
Recently, some of these problems have been mitigated using inductively coupled flat coils in combination with power electronics, which were ignored in the other earlier techniques. Several systems have been proposed for using magnetic induction to transfer power from a fixed source to a moving vehicle. For example, the following three articles propose various systems having a fixed primary winding buried in a roadway along which electric vehicles travel: K. Lashkari, S. E. Schladover, and E. H. Lechner, "Inductive Power Transfer to an Electric Vehicle," 8th International Electric Vehicle Symposium, Washington, D.C., October, 1986; E. H. Lechner and S. E. Schladover, "The Roadway Powered Electric Vehicle-An All-electric Hybrid System," 8th International Electric Vehicle Symposium, Washington, D.C., October, 1986; and S. E. Schladover, "Systems Engineering of the Roadway Powered Electric Vehicle Technology" 9th International Electric Vehicle Symposium, Toronto, Ont. Canada, November, 1988. However, these systems suffer a variety of drawbacks, including the need for embedded coils along the path of travel, mandatory fast and accurate air gap control, and the requirement of two large flux collection or conduction surfaces. Each of these drawbacks seriously impacts on the economic practicality of these systems.
As a specific example, the State of California is considering an electric vehicle highway proposal for contactless distribution of power to moving vehicles. In the California approach, cables are buried in the roadway and energized. The vehicle carries an induction coil which receives induced current for use in propulsion and battery recharging. The California system uses a positioning control on the vehicle to maintain the distance between the two flux collection surfaces of the buried cable and the coil to within 5 cm to provide sufficient power pickup.
The California system has several drawbacks. For example, the normal attractive forces between the vehicle and the buried cable can reach high levels during the required power transfer. The attractive forces also increase the frictional forces required to move the vehicle along the roadway. The controller must counter these attractive forces by positioning the vehicle-mounted collector above the roadway surface. Additionally, the magnetic coupling between the cable and coil in the California system is poor due to the large air gap, yielding poor efficiency, low power/weight density (i.e., kilowatts per kilogram), and poor utilization of core material. To compensate for the large air gap space between the vehicle-mounted secondary collector and the buried primary conductor a very large primary converter is required to power the buried conductor. To improve the magnetic coupling, the roadway cable must be buried with significant amounts of magnetic core material, which greatly increases the initial and operating costs of the California system.
Thus, a need exists for an improved manner of delivering power from a source to an electric load, especially where there is relative motion between the source and the load, such as may be used in industry, construction, mining, and electrical vehicle applications, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.